Solar cells fabricated from p-n junctions of boron nitride nanotubes alloyed with carbon are described. Band gaps of boron nitride carbon alloys are tailored by controlling carbon content in the boron nitride nanotubes. High efficiency solar cells can be fabricated by tailoring the band gap of boron nitride carbon alloy nanotubes, and using these nanotubes for fabricating solar cells u. Because boron nitride carbon alloy nanotubes are transparent to most wavelengths of light, the wavelengths not converted to electrons (i.e., absorbed) at a first p-n junction in a solar cell will pass through the stack to another p-n junction in the stack having a different band gap. At each successive p-n junction, each of which has a different band gap from the other p-n junctions in the stack, more wavelengths of light will be converted into electricity. This dramatically increases the efficiency of solar cells.